This work centers on the high molecular weight, high mobility group (HMG) nonhistone chromatin proteins. We have previously shown that HMG-1 and HMG-2 occur in cultured hepatoma cells in much higher amounts than in adult rat liver and that both proteins possess preferential affinity for single-stranded DNA. We have recently discovered a three-domain organization of HMG-1 and HMG-2. Two of the domains are sequence homologous, globular, DNA-binding domains. The third domain is probably a random coil that binds to histones. The proteins are thus peculiarly adapted to interact through separate domains with both DNA and histones. We have also made major progress in two-dimensional gel electrophoresis of HMG proteins. In addition, we have obtained improved resolution of multiple forms and have demonstrated a requirement for removing histones from the HMG proteins before two-dimensional electrophoretic analysis.